This invention generally relates to a process and apparatus, and more specifically, the present invention is directed to a process and apparatus for preparing semiconductive devices and photoelectronic devices. In one embodiment the present invention is directed to an apparatus and process for the preparation of a photoresponsive device by the deposition of a film of amorphous silicon on a cylindrical substrate contained in a vacuum chamber, by subjecting an appropriate gas source material, such as a silane gas to decomposition in an electrical discharge subsequent to causing such a gas to flow in a crossward direction toward the cylindrical substrate or in a direction orthogonal to the cylinder substrate axis. The resulting coated substrate, which can be a drum of aluminum, containing on its surface the deposited amorphous silicon is useful as a photoconductor, or can be incorporated in a photoresponsive imaging device or photoconductor member in, for example, electrostatic imaging devices.
Electrostatographic imaging systems, particularly xerographic imaging systems are well known in the art. In these systems, generally a photoresponsive or photoconductor material is selected for forming the latent electrostatic image thereon. Examples of photoconductive materials include amorphous selenium, alloys of selenium, such as selenium tellurium, selenium arsenic, and the like. Additionally, there can be selected as the photoresponsive imaging member, various organic photoconductive materials, including for example, complexes of trinitrofluorenone and polyvinylcarbazole. Recently, there has been developed layered organic photoresponsive devices containing charge transport layers and photogenerating layers. Examples of charge transport layers include various diamines, while examples of photogenerating layers include trigonal selenium, metal and metal free phthalocyanines, vanadyl phthalocyanine, and the like.
While all of these materials are suitable for their intended purposes, there continues to be a need for improved photoreceptor members. Recently for example, there has been disclosed the use of amorphous silicon (a-Si:H) as a photoreceptor material. This material possesses a number of advantages in comparison to, for example, amorphous selenium-based materials in that amorphous silicon is of extreme hardness and will not crystallize over extended time periods, even at temperatures as high as several hundred degrees Centigrade. Additionally, amorphous silicon photoconductors have excellent photoelectric properties, high absorption coefficients through the visible region, and are relatively low in useful life cost in comparison to selenium photoconductors, for example. Moreover, amorphous silicon members are capable of ambipolarity as they can be xerographically charged and discharged either positively or negatively in various imaging systems. Further, amorphous silicon can be modified by adding various dopants thereto, such as boron and phosphorous, enabling this material to function as p or n type semiconductor devices. Also, amorphous silicon may be alloyed with other elements, such as germanium and tin for the purpose of providing a material which will be photosensitive in the infra-red region.
Amorphous silicon, which is classified as a tetrahedrally bonded amorphous semiconductor can be prepared by known thermal evaporation techniques, similar to those techniques selected for the preparation of selenium photoconductors. However, amorphous silicon prepared in accordance with such a process usually has a relatively high dark conductivity, about 10.sup.-3 ohm-cm.sup.(-1) thereby causing the disapearance of any charge from the surface thereof resulting in a substantially useless member for xerographic imaging purposes. Amorphous silicon prepared by the glow discharge of the gas silane results in a material having a much lower conductivity, such as below 10.sup.-9 ohm-cm.sup.(-1) reference, for example, the Journal of Electrochemical Society, Volume 116, page 77, (1969) Chittick et al., and the Journal of Noncrystalline Solids, Volume 3, pages 255-270, (1970) Chittick et al. Apparently, in these processes hydrogen atoms saturate dangling silicon bonds and remove the band gap states causing the Fermi level to move towards mid-gap.
It is believed that the dangling bonds intrinsically incorporated in amorphous silicon can be reduced, and to some extend controlled by the choice of film deposition conditions. These dangling bonds are apparently present in a density of sufficient magnitude to render films of amorphous silicon prepared by thermal evaporation techniques or sputtering processes unsuitable for semiconductor and photoelectronic purposes. For example, the resulting material cannot be successfully doped and used to produce p or n type operative devices. Further, intrinsic dangling bonds function as recombination sites, rendering the resulting film substantially useless for photoelectronic devices. In those situations wherein, for example, amorphous silicon films are prepared in the presence of a reactive gas, the undesirable localized states are removed from the band gap as a result of the intrinsic dangling bond defects being coordinated with fragments of the reactive gas. Examples of reactive gases selected for coordination with the intrinsic silicon dangling bonds include hydrogen gas and fluorine gas.
Accordingly, thermally evaporated amorphous silicon generally must contain for example, hydrogen or fluorine in order to be useful as a photoconductive member. Thus, the preparation of such a material requires processes and apparatus which are vastly different from those required for the familiar thermal evaporation techniques selected for the preparation of chalcogenides.
One known method for obtaining amorphous silicon materials is referred to in the art as the glow discharge process. In this process, the vapor deposition of a silane gas occurs by causing the gas to flow between two electrodes, one of which has a substrate contained thereon. As electrical power is applied to the electrodes, the silane gas decomposes into a reactive silicon hydrogen species, which will deposit as a solid film on both electrodes. The presence of hydrogen can be of critical importance since it tends to coordinate with the dangling bonds in the silicon in part, as the mono, di, and tri-hydrides, thereby serving to passivate these dangling bonds.
In another known process, amorphous silicon can be prepared by a sputtering technique, wherein a substrate is attached to one electrode, and a target of silicon is placed on a second electrode. These electrodes are connected to a high voltage power supply and a gas which is usually a mixture of argon and hydrogen is introduced between the electrodes to provide a medium in which a glow discharge, or plasma can be initiated, and maintained. The glow discharge provides ions which strike the silicon target, and cause the removal by momentum transfer of mainly neutral target atoms, which subsequently condense as a thin film on the substrate electrode. Also, the glow discharge functions to activate the hydrogen, causing it to react with the silicon, and be incorporated into the deposited silicon film. The activated hydrogen also coordinates with the dangling bonds of the silicon to form mono, di, and trihydrides.
There is also known, as described in a copending application an apparatus and process for preparing amorphous silicon films on a substrate, which involves means for directing and accelerating an ion beam from a plasma toward a sputtering target contained within a chamber, which chamber also contains a shield means having a low sputtering efficiency compared to the sputtering target. The shield means is situated between stray ion beams and the vacuum chamber surface. More specifically, the ion beam process involves producing semiconductive films on a substrate comprising generating the plasma, directing and accelerating an ion beam of the plasma toward a sputtering target, the target being contained in a vacuum chamber at reduced pressure, shielding the vacuum chamber surface from stray ion beams, whereby sputtering of the vacuum chamber surface by the plasma is minimized, followed by sputtering the target with the ion beam to sputter the target material, collecting the sputtered target material as a film on the substrate, the substrate being physically isolated from the plasma generating process and the sputtering process.
While the latter processes may be suitable for their intended purposes, they suffer from a number of disadvantages, including for example, very low rates of material deposition, the inability to obtain uniform coatings over large areas, and the inability to form multi-layers without moving or removing the target and/or the substrate.
Additionally, there is disclosed in U.S. Pat. No. 4,265,991 an amorphous silicon photoconductor. This patent describes several processes for preparing amorphous silicon. In one process, there is prepared an electrophotographic photosensitive member which involves heating the electrophotographic member contained in a chamber to a temperature of 50.degree. C. to 350.degree. C., introducing a gas containing a hydrogen atom into the deposition chamber, causing an electrical discharge in the space of the deposition chamber, in which a silicon compound is present, by electric energy to ionize the gas, followed by depositing amorphous silicon on the electrophotographic substrate at a rate of 0.5 to 100 Angstroms per second by utilizing an electric discharge while raising the temperature of the substrate, thereby resulting in an amorphous silicon photoconductive layer of a predetermined thickness.
There continues to be a need for improved processes and apparatus for obtaining uniform films of photoconductive amorphous silicon at increased deposition rates over large areas. Additionally, there continues to be a need for improved processes and apparatus for preparing amorphous silicon photoconductor members, which processes and apparatus are simple in design, economically attractive, and which are susceptible to a batch operation. Moreover, there continues to be a need for processes and apparatus for preparing amorphous silicon photoconductive structures having superior mechanical strength, improved chemical stability, and substantially no toxicity problems associated therewith. Further, there continues to be a need for improved processes and apparatus for obtaining amorphous silicon photoconductors wherein the thickness of the resulting film on a substrate will be axially and radially uniform over a variety of operating conditions. Also, there continues to be a need for improved processes and apparatus for obtaining amorphous silicon photoconductors wherein virtually all the silane source gas material is converted to amorphous silicon. Additionally, there continues to be a need for improved apparatus for the deposition of amorphous silicon photoconductors, wherein the apparatus can be easily and expeditiously cleaned between depositions to avoid a buildup of material in the deposition chamber. Finally, there continues to be a need for a process and an apparatus which are suitable for the deposition of amorphous silicon and related materials on numerous cylindrical substrates contained in one deposition apparatus, such as a vacuum chamber.